[Solved] In application.yml, you wrote:db: host: ${DB_HOST} port: ${DB_PORT:3306}At runtime, you get an error about DB_HOST but not about DB_PORT. Why? | Spring Boot

Spring Boot - Application Configuration

In application.yml, you wrote:

db:
  host: ${DB_HOST}
  port: ${DB_PORT:3306}

At runtime, you get an error about DB_HOST but not about DB_PORT. Why?

ADB_HOST is not set and has no default; DB_PORT has a default value

BDB_PORT is set but DB_HOST is ignored

CYAML syntax is invalid for DB_HOST

DSpring Boot does not support defaults in YAML

Step-by-Step Solution

Solution:

Step 1: Check environment variable usage with and without defaults
DB_HOST has no default, so if unset, causes error. DB_PORT has default 3306, so no error if unset.
Step 2: Understand error behavior
Missing DB_HOST causes unresolved placeholder error; DB_PORT uses default silently.
Final Answer:
DB_HOST is not set and has no default; DB_PORT has a default value -> Option A
Quick Check:
Missing env var without default causes error [OK]

Quick Trick: Always provide defaults or set env vars to avoid errors [OK]

Common Mistakes:

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